A cloud drifts across the sun, and the afternoon suddenly feels cooler. Hours later, an overcast sky can keep the night surprisingly mild. The same clouds can also slow heat’s escape into space. How can one cloud have such opposite effects?
Clouds are powerful regulators of climate. They cover roughly two-thirds of Earth and alter its energy flows. Whether a cloud cools or warms the Earth depends on its height, thickness, composition, location, and the time of day.
Why Clouds Can Act Like Giant Sunshades
Sunlight reaches Earth as mostly shortwave radiation. Bright cloud tops reflect some of this energy back into space before it can warm oceans or land. Scientists call a surface’s or object’s reflectivity its albedo, and clouds generally have a higher albedo than the dark sea beneath them.
Low stratocumulus clouds are effective sunshades. Their layers spread across cool oceans. Their shade helps cool the planet.
How Clouds Also Keep the Planet Warm
Earth does not only absorb energy; it emits energy upward as invisible infrared radiation. Clouds absorb some of this outgoing heat and emit infrared energy in different directions, including downward. This contributes to the natural greenhouse effect.
The cloud’s temperature matters. High clouds are often much colder than the surface beneath them, so they emit less infrared energy to space than the warmer surface would. Thus, cloudy nights often stay warmer.
Why High Clouds and Low Clouds Behave Differently
Thin, high cirrus clouds are made largely of ice crystals. They allow much sunlight through while reducing the escape of infrared energy, so their overall effect is often warming. Low, thick clouds commonly reflect enough sunlight that their cooling influence wins.
These patterns have exceptions. A deep thunderstorm cloud can reflect substantial sunlight and affect outgoing infrared radiation at the same time. Its total influence depends on altitude, thickness, and droplet size.
Do Clouds Cool or Warm Earth Overall?
When both competing effects are combined, today’s clouds produce a net cooling of the planet. On average, they reduce absorbed sunlight by roughly 50 watts per square meter and outgoing heat by about 30. The balance is approximately 20 watts per square meter of net cooling.
Scientists call this the cloud radiative effect. It compares energy flows with and without clouds. Importantly, today’s net cooling does not mean that future changes in clouds must slow global warming.
Why Cloudy Days Feel Cool and Cloudy Nights Feel Warm
During daylight, a thick cloud deck shades the ground, often lowering afternoon temperatures. At night there is no incoming sunlight for clouds to reflect, but clouds can still absorb and emit infrared energy. Thus, overcast nights are commonly warmer than clear ones.
This pattern also explains why deserts, which often have clear skies, can experience dramatic drops in temperature after sunset. Humidity, wind, and local geography matter too. Clouds are only part of that exchange.
How Tiny Particles Change Cloud Brightness
Cloud droplets usually form around microscopic airborne particles called cloud condensation nuclei. These particles may come from sea salt, dust, or pollution. When many suitable particles are present, the available water can spread among more, smaller droplets.
Such clouds can become more reflective, a phenomenon visible in the bright tracks sometimes left in marine clouds by ship emissions. This is an example of aerosol–cloud interactions. Particles also affect cloud lifetime and rainfall.
What Happens to Clouds as the Climate Warms?
Warmer air can hold more water vapor, but that does not guarantee more clouds everywhere. Cloud formation also depends on air circulation, humidity, atmospheric stability, and rising or sinking motions. Some low-cloud regions may lose cloud cover, increasing heating.
High clouds may also rise as the atmosphere warms, strengthening their greenhouse influence. Together, these changes produce a cloud feedback: a response that alters the original temperature change. The Intergovernmental Panel on Climate Change assesses the overall feedback from clouds as amplifying global warming, although its size remains an important scientific question.
How Satellites Reveal the Hidden Role of Clouds
Ground observers see only the sky above them. Satellites view cloud fields across oceans, continents, and remote polar regions, measuring their extent, brightness, and altitude. Instruments in NASA’s Clouds and the Earth’s Radiant Energy System (CERES) measure reflected sunlight and outgoing thermal energy.
These measurements help scientists test climate models and estimate Earth’s energy budget. Radar and lidar reveal hidden cloud layers. Each observation improves our understanding of how cloud changes affect planetary temperature.
Could Humans Make Clouds Brighter to Cool the Earth?
Researchers have proposed marine cloud brightening, which would introduce tiny sea-salt particles into certain low marine clouds to increase their reflectivity. The proposal draws on the same physics that makes some ship tracks unusually bright. But predicting changes in rainfall, circulation, and regional climate remains difficult.
Cloud engineering is not an established substitute for cutting greenhouse gas emissions. Cloud changes in one region could have unintended effects elsewhere. Understanding natural clouds is the essential first step.
Clouds constantly change, yet profoundly shape planetary temperature. Next time you notice a bright cloud blocking the sun or a blanket of gray clouds overhead at night, which effect will you think of first: cooling or warming? Share your observations in the comments.
Interesting Facts
- Clouds typically cover about 67% of Earth’s surface at a given time.
- The present-day global cloud radiative effect is a net cooling of approximately 20 watts per square meter.
- Thin, high cirrus clouds often warm the climate, while thick, low clouds generally cool it.
- Clouds can make nights milder by reducing the rate of infrared energy loss from the surface.
- Ship exhaust can produce bright cloud trails by changing the number and size of cloud droplets.
- Researchers expect the combined response of clouds to increasing global temperatures to amplify warming overall.
Glossary
- Albedo — The proportion of incoming sunlight reflected by a surface or object.
- Stratocumulus — Low, often extensive clouds made of rounded patches or layers.
- Infrared Radiation — Invisible electromagnetic radiation emitted by Earth and other objects according to their temperature.
- Greenhouse Effect — Warming caused when atmospheric gases and clouds absorb and reemit outgoing infrared energy.
- Cirrus — Thin, high clouds composed mainly of ice crystals.
- Cloud Radiative Effect — The change in Earth’s energy flows associated with the presence of clouds.
- Cloud Condensation Nuclei — Tiny particles on which airborne water vapor can condense to form cloud droplets.
- Aerosol–Cloud Interactions — Ways airborne particles influence cloud formation, properties, and behavior.
- Cloud Feedback — Changes in clouds that either amplify or reduce an initial climate change.
- Energy Budget — The balance between energy entering, leaving, and accumulating in the Earth system.
- Marine Cloud Brightening — A proposed technique for increasing the reflectivity of some marine clouds.
